In many cases, it is desirable that optical transmission apparatuses transmit an optical signal at a predetermined target output level that is within a reception dynamic range of an optical receiver. To meet such a demand, the optical transmission apparatus has a function of dynamically controlling the gain of an optical amplifier in accordance with the power of an input optical signal (hereinafter referred to as an “output level control function”).
Through the output level control function, a drive voltage used for controlling the gain of an optical amplifier is generated in accordance with the optical input power. At that time, in order to provide high-speed response, the output level control function is realized using a configuration that employs, for example, a lookup table. In such a case, the lookup table is generated on the basis of the properties of the optical amplifier in advance. For example, the lookup table stores the correspondence relationship between an optical input power and a drive voltage of the optical amplifier for a predetermined target output level. Thereafter, through the output level control function, a drive voltage corresponding to a given optical input power is acquired from the lookup table, and control is performed on the gain of the lookup table using the acquired drive voltage. In this manner, the output level of the optical amplifier can be maintained at the target output level.
As a related art, a semiconductor optical amplifier apparatus having an output control function has been developed. In addition, an output beam intensity control apparatus that controls the intensity of an output beam so that the intensity is maintained at a certain level has been developed. Furthermore, a method for performing more flexible control by outputting data in a generated table and performing linear interpolation on the data has been developed (refer to, for example, Japanese Laid-open Patent Publication Nos. 2004-179233, 2010-10614, and 4-84056).
The above-described lookup table is generated by measuring, for each of the target output levels, the drive voltage at which the target output level is obtained while sweeping the optical input power. At that time, the target output level varies on a user-by-user basis. Accordingly, to satisfy the demand of each user, a plurality of lookup tables are generated in advance. In addition, to increase the dynamic range of the optical input power, the measurement is performed for each of a plurality of target output levels. Thus, in order to generate a general-purpose lookup table, a time for measurement is increased and, therefore, the cost for generating the lookup table is increased.
Such an issue can be addressed by, for example, reducing the number of measurement points when the lookup table is generated and computing data corresponding to predetermined target output levels using linear interpolation. However, in widely used linear interpolation, an error in the computed data (e.g., the drive voltage of the optical amplifier corresponding to the optical input power) increases. In such a case, the optical output level may be shifted from the target output level. In addition, depending on the properties of an optical detector that detects the optical input power and/or an optical amplifier that amplifies the drive voltage, a region in which an element of the lookup table is difficult to compute using linear interpolation appears. That is, for a desired target output level, it is difficult to obtain a correspondence relationship between the optical input power and the drive voltage of the optical amplifier. In such a case, the dynamic range of the optical input power is reduced, or the accuracy of control of the optical output level is reduced.